1. Field of the Invention
The present invention relates to a negative electrode of a power storage device and a power storage device having the negative electrode.
2. Description of the Related Art
With an increasing concern for the environmental issues, power storage devices such as secondary batteries and electric double layer capacitors used for power supply for hybrid vehicles and the like have been actively developed. As the power storage devices, lithium (Li)-ion secondary batteries and Li-ion capacitors which have high energy performance have attracted attention. The Li-ion secondary battery, which is compact but can have a large capacity, has already been mounted on a portable information terminal such as a mobile phone or a laptop personal computer, thereby contributing to miniaturization of products.
The power storage device basically has a structure in which an electrolyte is provided between a positive electrode and a negative electrode. It is known that each of the positive electrode and the negative electrode includes a current collector and an active material provided over the current collector. For example, in a Li-ion secondary battery, a material capable of storing and releasing Li ions is used as an active material.
Various approaches have been taken to improve the characteristics of a power storage device. For example, study of a negative electrode active material for a power storage device is one of the approaches to improve the characteristics of a power storage device. A graphite-based carbon material, which is mainly used as the negative electrode active material, has the theoretical capacity of 372 mAh/g and has already been put to practical use with a capacity close to the theoretical capacity. Thus, an active material with a higher capacity (charge capacity) is required.
A material containing a semimetal, a semimetal compound, a metal, or a metal compound is given as an example of a material having a higher capacity than a graphite-based carbon material when it is used as a negative electrode active material for a power storage device. For example, silicon (Si) is known to have a higher capacity than a graphite-based carbon material. Patent Document 1 discloses a negative electrode of a Li-ion secondary battery in which a fiber shaped carbon material, silicon, and a silicon compound are used in addition to a graphite-based carbon material. Patent Document 2 discloses a Li-ion secondary battery in which a graphite-based carbon material and a metal-carbon composite material are used.
However, a negative electrode active material with a higher capacity is required to meet an increasing demand for a compact power storage device.